This invention relates to the transference and compression of process gas containing impurities as for example in dual pressure plants for production of nitric acid and caprolactam. More specifically, the invention concerns an improved method for removing and preventing the formation of crystalline ammonium nitrate deposits in nitrous gas compressors and in other process streams where such deposition occurs.
The ammonium nitrate deposits which are formed, for instance by unreacted ammonia from catalytic oxidation of NH.sub.3 to NO, will in particular reduce the flow capacity of nitrous gas compressors, increase power consumption and may cause imbalance in the rotating parts of the compressors. Furthermore, such salt deposits may represent a safety hazard if the accumulation of salt is not prevented or restricted.
From the days when the first nitrous gas compressors were installed in nitric acid plants over 30 years ago, it has been the practice to utilize injection of and spraying with water to remove the salt deposits. The compressors are usually built with rows of spray nozzles in the flow channels for periodic water injection during plant operation. The time intervals between each washing operation may vary from 4 to 36 hours and the time for the washing operation may vary between 10 and 30 minutes. Normal water addition during washing will be in the region of 0.5-2.0 gram/kmol process gas. Some nitrous gas compressors even have continuous water injection in addition to discontinuous washing.
Water is also normally added continuously to the sealing systems of the compressors to prevent fouling of the labyrinths caused by salt deposits. This water injection which may be up to 500 kg/h, ends up in the product, either by evaporation in the process gas or by drainage as condensate.
Thus, significant amounts of water are used to keep the salt deposits under control. Such addition of wash water to the process gas or to the condensate is, however, undesirable, as the water has to be compensated by a corresponding reduction in the process water to the absorption system, resulting in a reduction in absorption efficiency or in the maximum attainable product concentration.
In spite of prolonged water injection into the compressor, washing does not remove sufficient deposit to restore maximum capacity. This is due to the fact that because of their inertia the injected water droplets do not moisten or humidify all surfaces where deposits occur. In some compressors the salt accumulation on these surfaces may be so extensive that it will be necessary to stop the compressor at intervals for more thorough washing to restore the original capacity.
The injected water droplets may also lead to serious erosion in the compressor, especially on the rotor blades of axial compressors and on the riveted connections in centrifugal compressors.
Furthermore, more efficient removal of the salt deposits is especially desirable to maintain a higher average flow capacity in the compressor, in order to increase the production of the plant.
Thus, it is a main object of the invention to provide a new and improved method for the removal of salt deposits avoiding the above mentioned serious drawbacks.